Joel Joffe had just arrived on British shores, after being "exited" from South Africa, when he saw a leaflet offering life membership for the Voluntary Euthanasia Society for just 20 guineas.

The human rights lawyer, who once counted Nelson Mandela among his clients, decided to sign up. Forty years on, and now a cross-bench peer, his name has become inextricably linked with the controversial cause of legalising people's right to be helped to die.

Though he has a track record in the charity sector, Lord Joffe's energies for the past three years have been concentrated on this single issue.

"I have really not focused on anything else", he says with a smile. "It has taken up more of my time than I would care to mention."

He is confident that the issue is gathering momentum, not least because of the infamous struggle of motor neurone disease sufferer Diane Pretty, who fought an unsuccessful battle in the European Court of Human Rights to be helped to die by her husband Brian.

Pretty wanted to die before she faced the swallowing and breathing difficulties that ensue in the last stages of the disease.

"The issue really has taken off and generated a valuable debate in an area which people preferred not to debate. I have been encouraged by the significant support we have received in the Lords," says Lord Joffe.

The peer's original private members' bill, first tabled in 2002, proposed enabling a competent adult who is suffering unbearably to receive medical assistance to die at his or own request.

Like most private members bills, introduced without government backing, it fell in June 2003. Lord Joffe tabled a revised version in the autumn of 2003, which tightened up eligibility to include only those who were terminally ill; that bill lapsed again in the following summer. A third attempt was thwarted by the general election, when the bill fell once more.

Lord Joffe remains undeterred, recalling last week's "incredible turnout" for an intense eight-hour debate session in the Lords. Seventy-five peers took their chance to speak on a report published by a special select committee set up to scrutinise the proposed legislation.

That committee's report, published last spring, called for a clear distinction between voluntary euthanasia - where a doctor hastens death at a patient's request - and physician-assisted suicide, where a doctor helps the patient to kill himself.

Lord Joffe's concession to the committee, as he prepares to table the bill for a fourth time, is to remove voluntary euthanasia from the bill. He nevertheless seems confident this would not result in a loophole for people such as Pretty, who are not able to self-administer drugs.

Technology now provides ways for people to ingest liquid lethal medication without much physical manoeuvring, he explains. "It should be possible to cover almost every case and it is most important that there should be a remedy for people who are physically unable."

But the peer will not concede ground on another select committee recommendation, namely that the definition of unbearable suffering be replaced with "unrelievable" or "intractable" suffering or distress.

Lord Joffe is aware that his views are not widely shared among faith groups. Born Jewish, he describes himself as an agnostic who believes in many "religious values".

"The person who must be the judge of suffering is the patient," says Lord Joffe, pointing out that feeling that something is 'unbearable' varies with the individual.

"The wish of doctors to treat everybody, regardless of whether they wish to be treated or not, is in my view inappropriate. The slogan now is 'patient choice'. If a doctor says 'I can help this person,' and the person does not want to be helped, that is a decision for them."

He counters claims made by the bill's detractors that many people with terminal illness may feel compelled to end their days prematurely to avoid being a further "burden" to their carers if the law were finally introduced.

"The key to it is that the sort of people who would ask for assisted dying are independent and used to making their own decisions in their own lives. You find carers mostly want the patients to carry on living, and the patient says: 'No, I don't want to be.' To pretend one is not a burden when one is dying of motor neurone disease is not facing up to reality.

"People are burdens and they do not want to be. The carers are the people who want to keep them alive. But it is not for the carers to decide. It is for the individuals.

"The underlying principle on which the whole bill is based upon is personal autonomy and people making decisions for themselves. "

He is bewildered by the "contradiction" within sections of the disability lobby, some of whom fear that the law will be used to discriminate against disabled people's quality of life and persuade them to end their lives instead. He returns to the fact that the law is only intended for terminally ill people, not those living with disability.

"Disabled people who have lived with disability all their lives and fight for equal rights are in a better position to withstand pressure. They have been discriminated against throughout their life. They have not capitulated to the pressure; why should they suddenly cap in the last six months of their lives?"

What is most likely to persuade people to see out the end of their lives naturally, the peer says, is good palliative care. He is concerned by the current levels of provision across the country, which he says must be improved. "Sometimes people in pain are suffering and say 'I want to die,' but it is not a serious request", he says. "It is a cry for help, and by pushing them in the direction of palliative care this would normally provide the answer.

"I think palliative care is the right answer for the overwhelming majority of cases because have an incredible will to live. I am always surprised they do. If I felt this would be harming palliative care I don't think I would be proceeding with the bill.

"I strongly support the case for very significant expansion of government funding for hospice and the palliative care movement", he adds emphatically.

What Lord Joffe is trying to provide is choice for those who don't want palliative care at all. His sights are set on the Oregon model in the US, where one in every 700 terminally ill patients chooses assisted dying over care.

"Some people do not want palliative care. They say: 'I have had a good life and I don't want to spend the last three to four months of my life suffering unbearably or being drugged. It is my right to determine how and when I die.' It is for those people the bill is intended."

Aware that private members bills which begin in the Lords almost never make it to statute without the government's blessing, Joffe remains "cautiously optimistic" that the government has finally pricked up its ears to the debate. Poll after poll shows that the overwhelming majority of the general public supports assisted dying laws, though this finding is not reflected in the Commons among elected MPs.

"The government is taking a neutral stance - but as Lord May pointed out in the debate the other day on October 10, it is inconceivable that the government would not make the time available, bearing in mind the space the government gave to the preservation of foxes."